test: improve coverage of changelog, semver, cfg and vcs core logic

[outofcoffee]

Add targeted tests for previously untested core logic across the changelog, semver, cfg and vcs packages.

Summary

• Cover NoChangesError.Error across all four message branches
• Add changelog read error paths (missing version, no version sections, non-existent file) and the last-section-to-end-of-file boundary
• Test cfg.LoadConfig directory wrapper and its missing-file behaviour
• Verify semver.GetCurrentVersion strips the v prefix and sets the prefix flag
• Cover vcs.CheckBranch matching-branch and invalid-repo paths
• Add an end-to-end changelog.InitChangelog test asserting written content matches disk

Implementation details

The vcs package caches the earliest/latest tag in package-level variables that other packages cannot reset, so the new semver test performs a single repository lookup to avoid cross-test contamination, and CheckBranch resolves the repo's actual current branch rather than hard-coding master/main. The last-section read test deliberately includes a trailing empty line, mirroring how ReadFile splits a file ending in a newline — readChanges relies on that to bound the final section. All tests pass under -shuffle=on.